Computed Tomography System and Method

ABSTRACT

A computed tomography system having a fixed X-ray source [ 10]  for producing a fan beam [ 20] , a fixed digital detector [ 12]  and a manipulator [ 14]  for holding and rotating an object [ 16]  to be inspected. Left and right projections of the rotated object on the fixed digital detector are used to determine a central ray, reconstruction of an image of the object being based on the central ray position. A corresponding method and apparatus are also disclosed.

FIELD OF THE INVENTION

This invention relates to a computed tomography system and method andrefers particularly, though not exclusively, to an X-ray computedtomography inspection system and method for industrial application. Moreparticularly, the invention relates to the determination of the centralray by use of projections of the object to be scanned

BACKGROUND OF THE INVENTION

A typical X-ray micro-computed tomography (“CT”) system for industrialapplications consists of an X-ray source, a manipulator/rotator forpositioning and rotating the object to be scanned, and an X-ray detector(camera). A good CT scan requires the accurate determination of thecentral ray. The central ray is sometimes called the iso-channel and isthe virtual projection of the center-of-rotation on the detector.

Most fan-beam micro-CT inspection systems are also equipped withtwo-dimensional inspection as their basic capability. These systemsfrequently require that the manipulator move from place-to-place; andthat the rotator be mounted on, and removable from, the manipulator. Theconsequence of this flexibility is the prerequisite determination of thecentral ray each time the manipulator or the rotator is moved. Even withthe same manipulator coordinates, generally the system will give twocentral ray positions that may be sufficiently different to beunacceptable.

A common solution to this problem is to use a wire phantom to calibratethe central ray position before each CT scan. To do so, the wire phantomis placed on the rotational axis and is rotated for 360 degrees inpredetermined angular steps. The projections of the wire phantom at allangles are then recorded and used for the determination of the centralray. The wire phantom is usually quite small so that it can be treatedas a point for all angles. With fan-beam geometry, due to the smalldeviation of the wire phantom to the axis of the rotation, the centralray is simply determined as being center of the sinogram of the wirephantom.

The use of a wire phantom to determine the central ray position createsmany problems including, but not limited to: reducing the speed of theCT process; introducing errors when changing the object for the wirephantom due to different weight, different fixing status, and so forth,thereby affecting the final CT image quality; and CT scans cannot beautomated. Large errors result when large magnification is needed, andthe object has to be placed close to the source. In this case, themanipulator is required to move away from the source so that there isspace for changing the object for the wire phantom and vise versa.

SUMMARY OF THE INVENTION

In accordance with a first preferred aspect there is provided a computedtomography system comprising:

(a) a fixed X-ray source for producing a fan beam;

(b) a fixed digital detector;

(c) a manipulator for holding and rotating an object to be inspected;

wherein left and right projections of the rotated object on the fixeddigital detector are used to determine a central ray position,reconstruction of an image of the object being based on the central rayposition.

According to a second preferred aspect there is provided a computedtomography method comprising:

(a) producing a fan beam of X-rays at a fixed X-ray;

(b) detecting the X-rays at a fixed digital detector;

(c) rotating an object to be inspected using a manipulator;

(d) determining left and right projections of the object on the fixeddigital detector;

(e) determining a central ray position from the left and rightprojections; and

(f) reconstructing an image of the object using the central rayposition.

A sinogram of the projections of the object may be used to determine thecentral ray position. The central ray may be determined by identifyingthe left and right ends of the sinogram.

An included angle between the left projection of the object, the fixedX-ray source, and the right projection of the object, may be used todetermine the central ray. The central ray may bisect the includedangle.

A part of the object with a largest radius to an axis of rotation isused to determine the left and right projections of the object on thefixed digital detector, the left projection of the part being a leftmostprojection and the right projection being the rightmost projection.Alternatively, the left and right projections of a point of the objectwhich generate a much clearer contrast may be used to determine thecentral ray position. The point may comprise a relatively small objectmade of a material more dense than that of the object; the relativelysmall object being attached to the object. The relatively small objectmay be attached to the object remote from at least one area of interestof the object for enabling a reconstructed image quality to not beaffected.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be fully understood and readilyput into practical effect, there shall now be described by way ofnon-limitative example only preferred embodiments of the presentinvention, the description being with reference to the accompanyingillustrative drawings.

In the drawings:

FIG. 1( a) is a schematic representation of a typical prior art X-raymicro-CT system for industrial application;

FIG. 1( b) illustrates a prior art CT scan and reconstruction system;

FIG. 2 is a flowchart of a prior art CT scan process with the CT systemof FIGS. 1( a) and 1(b);

FIG. 3( a) is a schematic diagram of a prior art CT system using a wirephantom for central ray determination;

FIG. 3( b) is the photograph of a practical wire phantom used forcentral ray determination;

FIG. 3( c) is an example of a sinogram of a prior art phantom wiregenerated from its projections;

FIG. 4( a) illustrates a first preferred embodiment in which an objectwith several balls of different radius is used for a CT scan;

FIG. 4( b) is a flowchart of the CT scan process of the first preferredembodiment;

FIG. 5( a) shows the central ray of the first preferred embodimentdetermined with a wire phantom at a randomly chosen rotation axis at adistance to the central channel;

FIG. 5( b) shows the central ray directly determined with objects withbig differences in both shape and materials, all objects being scannedat the same rotation axis as FIG. 5( a);

FIG. 6( a) is a demonstration with a pen-shaped diamond cutter cap;

FIG. 6( b) is a 2D projection of the object of FIG. 6( a);

FIG. 6( c) is a reconstructed image of the object of FIG. 6( a);

FIG. 7( a) is a demonstration with a carved ceramic owl;

FIG. 7( b) is a 2D projection of the object of FIG. 7( a);

FIG. 7( c) is a reconstructed image of the object of FIG. 7( a);

FIG. 8( a) is a demonstration with a walnut;

FIG. 8( b) is a 2D projection of the object of FIG. 8( a);

FIG. 8( c) is a reconstructed image of the object of FIG. 8( a);

FIG. 9( a) is a demonstration with aluminum foam;

FIG. 9( b) is a 2D projection of the object of FIG. 9( a);

FIG. 9( c) is a reconstructed image of the object of FIG. 9( a);

FIG. 10 is a view corresponding to FIG. 4( a) but of a second preferredembodiment; and

FIG. 11 is a view corresponding to FIG. 4( a) but of third preferredembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The CT scan of the object to be inspected is performed over 360 degrees,or over an arc of 180 degrees plus the fan-beam angle, at selectedangles. The projections at each angle are recorded. These projectionsare then used for both central ray determination and imagereconstruction. The principle behind the method is that the left end andthe right end of the sinogram of a selected object slice come from thepoint on the object which has the largest radius from the rotation axisof the object. The central ray bisects the angle formed by the two endpoints and the X-ray source point. Therefore, with prior knowledge ofthe central channel (which is always fixed) the central ray can bedetermined by a geometric relationship. The central ray may bedetermined simply as the center of the left and right ends of thesinogram if high accuracy is not required, or the left and right endsidentified are close to each other, or the angles formed by the leftprojection and right projection with respect to the central channel areboth small.

FIG. 1( a) shows a schematic representation of a typical prior artmicro-CT system. It consists of a fixed X-ray source, a fixed flat panelX-ray detector 12 and a manipulator 14 with a rotator 18 for holding,moving and rotating an object 16 to be inspected. The manipulator 14 maybe a high precision positioning stage that can move at least in the x, yand/or z directions; and the rotator 18 can rotate about an axis that isaligned to be parallel to one dimension of the detector array. An X-rayfan beam 20 is generated from the X-ray source 10, passing through theobject 16 and projecting on the detector 12. The magnification of thesystem is determined by the source-to-object distance 30 (SOD) (i.e.from 10 to 16) and the source-to-image 32 (SID) distance (i.e. from 10to 12).

FIG. 1( b) illustrates the arrangement for a CT scan. The central ray(iso-channel) 22 is defined as the projection of the rotation axis 44 onthe detector 12. The central channel 24 is the detector cell that the Xray projects on and which is perpendicular to the plane of the detector12. The pixel size 26 is the size of each cell 28 of detector 12.

FIGS. 2 and 3( a) show a typical CT scan using such a prior art micro-CTsystem. Before the CT scan, the object 16 to be inspected is mounted onthe rotator 18 and the best scan position for the object 16 isidentified (201). The proper tube voltage and tube current for a goodcontrast of the image for all scan angles are selected. The object 16 isthen removed and calibration for offset (202), gain (203) and wedge(204) conducted. A wire phantom 34 is mounted (205). The wire phantom 34is usually a straight wire fixed in a plastic tube. It is held by therotator 18 and rotated for 360-degree at predetermined angle stepsduring which it is scanned for central ray 22 determination (206). Thesinogram is then generated from all the inspections obtained at allangles and is used to determine the position of the central ray 22. Ifthe central ray 22 is not successfully identified, a larger diameterwire phantom 34 is used until the central ray 22 is determined. Afterthe successful determination of the central ray 22, the wire phantom 34is removed and the object 16 to be inspected is again mounted on therotator 18 (207). The corresponding CT scan parameters are set (208) andthe CT scan commenced (209). With the inspection of the object 16obtained, the cross-sectional image of the object 16 is reconstructed(240).

FIG. 3( b) shows a photo of a practical wire phantom and FIG. 3( c) isan example of the sinogram of a wire phantom generated from itsinspections.

FIG. 4 illustrates the principle of the first embodiment in which anobject 16 with four balls of different radius from the rotation axis 44is used for a CT scan. With a scan of a complete circle, or an 180degrees plus the fan beam angle arc, only the ball 36 with the largestradius generates the widest projection on the detector 12. That is, theleft 40 and right 42 boundaries of the sinogram of a selected slice arefrom the largest-radius ball 36 in that slice of the object 16.Therefore, once the position of the rotation axis 44 is given, theincluded angle of:

the left tangential point M, where the beam that provides the leftmostpoint 40 creates a tangent with ball 36, the source 10, and

the right tangential point N where the beam that provides the rightmostpoint 42 creates a tangent with ball 36 (“MSN”) is determined by usingthe radius of the largest-radius ball 36. By finding the correspondingscan angles of the left 40 and right 42 boundaries of the projection,the MSN angle can be determined. Because the central ray 22 must bisectthe MSN angle, the angle for the central ray 22 is obtained. The left 40and right 42 boundaries can be identified by use of a known edgedetection algorithm. Additional methods such as curve fitting may beused to improve the accuracy to a sub-pixel level.

One method to determine the MSN angle is to make use of the knowncentral channel 24 and the detector pixel size 26. As described before,the central channel 24 is defined as the ray perpendicular to thedetector array. With a fixed X-ray source 10 and detector 12, thecentral channel 24 and detector pixel size 26 are always fixed and willnot change unless the source 10 and/or detector 12 are moved. This maybe due to, for example, replacing a damaged camera. Based on a knowncentral channel C and detector pixel size p, the central ray can becalculated as following:

$\overset{\_}{OC} = {\left( \frac{\overset{\_}{SC}}{p} \right)*{tg}\left\{ {\left\lbrack {{{tg}^{- 1}\left( \frac{\overset{\_}{LC}*p}{\overset{\_}{SC}} \right)} - {{tg}^{- 1}\left( \frac{\overset{\_}{CR}*p}{\overset{\_}{SC}} \right)}} \right\rbrack/2} \right\}}$

where SC is the source-to-detector distance (unit: μm); OC, LC, RC arethe distances from the central ray point O, the left end of projection Land the right end of projection R to the central channel point Crespectively (unit: pixel). All SC, OC, LC, RC are vectors and their thesigns are determined according to their relative positions to thecentral channel.For circumstances where

${{{tg}^{- 1}\left( \frac{\overset{\_}{LC}*p}{\overset{\_}{SC}} \right)} \approx {\frac{\overset{\_}{LC}*p}{\overset{\_}{SC}}\mspace{14mu} {and}\mspace{14mu} {{tg}^{- 1}\left( \frac{\overset{\_}{CR}*p}{\overset{\_}{SC}} \right)}} \approx \frac{\overset{\_}{CR}*p}{\overset{\_}{SC}}},$

the above formula can be simplified as following

$\overset{\_}{OC} = {{\frac{1}{2}\left( {\overset{\_}{LC} - \overset{\_}{CR}} \right)} = {\frac{1}{2}\left( {{{LC}} + {{CR}}} \right)}}$

With the central ray point O identified, the process of reconstructioncan start.

FIG. 4( b) is a flowchart of the CT scan. The object 16 is mounted andthe parameters set (401). After shifting the manipulator with the object16 sideways out of the radiation area, calibrations for offset (402),gain (403) and wedge (404) effects take place. With a stable X-raysource 10 and digital detector 12, this step can be conducted once a dayand only needs to be repeated when the tube voltage or current ischanged. The manipulator with the object 16 is shifted back to theprevious position (405) on the rotator 18 and the scan (406) andreconstruction (407) take place.

Based on this CT scan, a one-step micro industry CT inspection system ispossible that is simpler than previous systems, and enables theautomatic performing of determining the system parameter settings,object positioning, offset calibration, gain calibration, wedgecalibration, the CT scan, and image reconstruction. The automated systemparameter setting may be by image analysis of the object underillumination, or checking a look-up table created for the relationshipbetween the system parameters and the object's properties, includingshape and size. Automated object positioning may be achieved byanalyzing the image of the object under illumination.

FIG. 5( a) shows the central ray 22 determined with a wire phantom 34 ata randomly chosen rotation axis. The wire phantom 34 is intentionallyplaced at a distance to the rotational axis to create a radius to therotational axis.

FIG. 5( b) summarizes the central rays determined with objects withdifferences in both shape and material. All objects were scanned at thesame rotation axis as FIG. 5( a). The values of the central rays areproperly determined using their own projection data. The variationsagree with that observed with a wire phantom 34 through a repeatabilitystudy.

FIG. 6 is a demonstration with a pen-shaped diamond cutter cap. FIG. 6(a) is a 2D projection of the object; FIG. 6( b) is a sinogram of oneslice with the left end and right end of the sinogram identified; andFIG. 6( c) is the reconstructed slice image of the object.

FIG. 7 is a demonstration with a carved ceramic owl. FIG. 7( a) is a 2Dprojection of the object; FIG. 7( b) is a sinogram of one slice with theleft end and right end of sinogram identified; and FIG. 7( c) is thereconstructed slice image of the object.

FIG. 8 is a demonstration with a walnut. FIG. 8( a) is a 2D projectionof the object; FIG. 8( b) is a sinogram of one slice with the left endand right end of sinogram identified; and FIG. 8( c) is thereconstructed slice image of the object.

FIG. 9 is the demonstration with aluminum foam. FIG. 9( a) is a 2Dprojection of the object; FIG. 9( b) is a sonogram of one slice with theleft end and right end of sinogram identified; and FIG. 9( c) is thereconstructed slice image of the object.

FIG. 10 is a second embodiment in which a relatively small object 50made of a dense material is adhered to the object 16 to be inspected,which has a special structure or material density distribution. Thesmall object 50 may be placed at a non-interest area of the object 16 sothat the areas of interest of the object 16 will not be affected. The CTscan can then be performed in the manner as described above. Because ofthe special structure or material property distribution of the object16, it may not generate good projection contrast for proper boundaryidentification, the small object 50 is used to produce the necessarycontrast.

FIG. 11 shows a third embodiment with which the projection of the objectpoint with largest radius 36 is not used. The projection of the point 38which generates a contrast much clearer than its surrounding points isused for calculating the central ray 22.

The CT process is therefore simplified and more user-friendly. It isalso possible to integrate all calibration processes into a CT scanprocess so automation is improved.

Whilst there has been described in the foregoing description preferredembodiments of the present invention, it will be understood by thoseskilled in the technology concerned that many variations ormodifications in details of design or construction may be made withoutdeparting from the present invention.

1. A computed tomography system comprising: (a) a fixed X-ray source forproducing a fan beam; (b) a fixed digital detector; and (c) amanipulator for holding and rotating an object to be inspected; andwherein left and right projections of the rotated object on the fixeddigital detector are used to determine a central ray position,reconstruction of an image of the object being based on the central rayposition.
 2. The system as claimed in claim 1, wherein a sinogram of theprojections of the object is used to determine the central ray position.3. A computed tomography system as claimed in claim 1, wherein anincluded angle is used to determine the central ray position, theincluded angle being between the left projection of the object, thefixed X-ray source, and the right projection of the object.
 4. Thesystem as claimed in claim 3, wherein the central ray bisects theincluded angle.
 5. The system as claimed in claim 3, wherein a part ofthe object with a largest radius to an axis of rotation is used todetermine the left and right projections of the object on the fixeddigital detector, the left projection of the part being a leftmostprojection and the right projection being the rightmost projection. 6.The system as claimed in claim 3 when appended to claim 2, wherein acentral channel and a pixel size of the detector are known; the centralray being determined by identifying the left and right ends of thesonogram.
 7. The system of claim 3, wherein left and right projectionsof a point of the object which generates a much clearer contrast areused to determine the central ray position.
 8. The system as claimed inclaim 7, wherein the point comprises a relatively small object made of amaterial more dense than that of the object; the relatively small objectbeing attached to the object.
 9. The system as claimed in claim 8,wherein the relatively small object is attached to the object remotefrom at least one area of interest of the object for enabling areconstructed image quality to not be affected.
 10. A computedtomography method comprising: (a) producing a fan beam of X-rays at afixed X-ray source; (b) detecting the X-rays at a fixed digitaldetector; (c) rotating an object to be inspected using a manipulator;(d) determining left and right projections of the object on the fixeddigital detector; (e) determining a central ray position from the leftand right projections; and (f) reconstructing an image of the objectusing the central ray position.
 11. The method as claimed in claim 10,wherein a sinogram of the projections of the object is used to determinethe central ray position.
 12. A method as claimed in claim 10, whereinan included angle is used to determine the central ray position, theincluded angle being between the left projection of the object, thefixed X-ray source, and the right projection of the object.
 13. Themethod as claimed in claim 12, wherein the central ray bisects theincluded angle.
 14. The method as claimed in claim 12, wherein a part ofthe object with a largest radius to an axis of rotation is used todetermine the left and right projections of the object on the fixeddigital detector; the left projection of the part being a leftmostprojection, and the right projection of the part being the rightmostprojection.
 15. The method as claimed in claim 12 when appended to claim11, wherein a central channel and a pixel size of the detector areknown; the central ray being determined by identifying the left andright ends of the sonogram.
 16. The method of claim 12, wherein the leftand right projections of a point of the object which generates a clearercontrast are used to determine the central ray position.
 17. The methodas claimed in claim 16, wherein the point comprises a relatively smallobject made of a material more dense than that of the object; therelatively small object being attached to the object.
 18. The method asclaimed in claim 17, wherein the relatively small object is attached tothe objection remote from areas of interest of the object for enabling areconstructed image quality to not be affected.